Measuring phase noise in radio frequency, microwave or millimeter signals based on photonic delay

ABSTRACT

Techniques and devices for measuring phase noise in radio frequency (RF), microwave, or millimeter signals based on photonic delay.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 12/889,398 filed Sep. 23, 2010, which claims the benefit of thefiling dates of U.S. Provisional Application No. 61/244,959 filed Sep.23, 2009 and U.S. Provisional Application No. 61/333,665 filed May 11,2010.

U.S. patent application Ser. No. 12/889,398 application is also acontinuation-in-part of U.S. patent application Ser. No. 12/270,845filed Nov. 13, 2008, which claims the benefit of the filing date of U.S.Provisional Application No. 61/002,918 filed Nov. 13, 2007.

The entire contents of the before-mentioned patent applications areincorporated by reference as part of the disclosure of this application.

BACKGROUND

This document relates to techniques and devices for measuring phasenoise in radio frequency (RF), microwave, or millimeter signals.

RF, microwave or millimeter signals can be generated by oscillatorsoperating in the respective spectral ranges. The output of an oscillatormay be used in communications and other applications. The oscillationfrequency of an oscillator can be used as a frequency reference where itis desirable that the noise of the oscillator be low and can be properlymeasured. A measurement apparatus for characterizing an oscillatorshould have low noise.

SUMMARY

This document describes techniques and devices for measuring phase noisein radio frequency (RF), microwave, or millimeter signals based onphotonic delay.

In one aspect, a device for measuring a phase noise in a signal isprovided to include an input port that receives an oscillation signalfrom an oscillator under test; a first photonic signal processing branchcircuit that processes the oscillation signal to produce a first branchoutput signal; and a second photonic signal processing branch circuitthat processes the oscillation signal to produce a second branch outputsignal. The first and second photonic signal processing branch circuitsshare a common optical module that includes a shared laser producingcontinuous wave laser light at first and second wavelengths, a sharedoptical modulator that modulates the laser light at the first and secondwavelengths to produce modulated laser light that carries theoscillation signal, a shared optical delay that receives the modulatedlaser light from the shared optical modulator and a wavelength-selectiveoptical device that separates the modulated laser light output by theshared optical delay into a first modulated laser beam at the firstwavelength that is processed by the first photonic signal processingbranch circuit to produce the first branch output signal and a secondmodulated laser beam at the second wavelength that is processed by thesecond photonic signal processing branch circuit to produce the secondbranch output signal. The device includes circuitry that receives thefirst and second branch output signals to measure noise in the receivedoscillation signal and controls the first and second photonic signalprocessing branch circuits and measurements of the noise in the receivedoscillation signal.

In another aspect, a device for measuring a phase noise in a signal isprovided to include an input port that receives an oscillation signalfrom an oscillator under test; a photonic signal processing circuit thatprocesses the oscillation signal to produce an output signal; andcircuitry that receives and processes the output signal to measure noisein the received oscillation signal and controls the photonic signalprocessing circuit and measurements of the noise in the receivedoscillation signal. The photonic signal processing circuit includes alaser producing continuous wave laser light in a first opticalpolarization, an optical modulator that modulates the laser light toproduce modulated laser light that carries the oscillation signal and isin the first optical polarization, a photonic beam combiner thatreceives at a first port the modulated laser light in the first opticalpolarization along a first optical path from the optical modulator,directs the received modulated laser light in the first opticalpolarization to a second port and directs light received at the secondport in a second optical polarization orthogonal to the first opticalpolarization to a third port, a fiber delay line coupled to the secondport to receive light from the photonic beam combiner, a Faraday rotatormirror coupled to the fiber delay line to reflect light back to thefiber delay line by rotating optical polarization by 90 degrees, aphotodetector coupled to receive light from the third port of thephotonic beam combiner to generate a detector signal, a voltagecontrolled phase shifter that receives a copy of the oscillation signaland changes a phase of the copy of the oscillator signal to produce aphase-shifted oscillator signal, and a signal mixer that mixes thedetector signal and the phase-shifted oscillator signal to produce theoutput signal.

In another aspect, a device for measuring a phase noise in a signal isprovided to include an input port that receives an oscillation signalfrom an oscillator under test; a first laser producing a firstcontinuous wave laser beam in a first optical polarization; a firstoptical modulator that modulates the first laser beam to produce a firstmodulated laser light that carries the oscillation signal; a firstoptical circulator having a first port that receives the first modulatedlaser light in the first optical polarization and a second port thatoutputs light from the first port and a third port that outputs lightreceived at the second port; a second laser producing a secondcontinuous wave laser beam in a second optical polarization orthogonalto the first optical polarization; a second optical modulator thatmodulates the second laser beam to produce a second modulated laserlight that carries the oscillation signal; and a second opticalcirculator having a first port that receives the second modulated laserlight in the second optical polarization and a second port that outputslight from the first port and a third port that outputs light receivedat the second port. This device includes photonic beam combiner thatincludes a first port, a second port and a third port. The first port iscoupled to the second port of the first optical circulator to receivethe first modulated laser light in the first optical polarization whichis directed to the second port of the photonic beam combiner. Thephotonic beam combiner directs light received at the second port in thesecond optical polarization to the third port and directs light receivedat the second port in the second optical polarization to the first port,and the third port of the photonic beam combiner is coupled to receivelight of the second modulated laser beam in the second polarization fromthe second port of the second optical circulator. This device includes afiber delay line coupled to the second port of the photonic beamcombiner to receive light from the photonic beam combiner to introduce aphase delay in both the first and second modulated laser beams; aFaraday rotator mirror coupled to the fiber delay line to reflect lightback to the fiber delay line by rotating optical polarization by 90degrees; a first photodetector coupled to receive light from the thirdport of the second optical circulator to generate a first detectorsignal; a second photodetector coupled to receive light from the thirdport of the first optical circulator to generate a second detectorsignal; a first voltage controlled phase shifter that receives a copy ofthe oscillation signal and changes a phase of the copy of the oscillatorsignal to produce a first phase-shifted oscillator signal; a firstsignal mixer that mixes the first detector signal and the firstphase-shifted oscillator signal to produce a first output signal; asecond voltage controlled phase shifter that receives another copy ofthe oscillation signal and changes a phase of the other copy of theoscillator signal to produce a second phase-shifted oscillator signal;and a second signal mixer that mixes the second detector signal and thesecond phase-shifted oscillator signal to produce a second outputsignal. This device also includes circuitry that receives the first andsecond output signals to measure noise in the received oscillationsignal, and controls the first and second voltage controlled phaseshifters and measurements of the noise in the received oscillationsignal.

In yet another aspect, this document provides an implementation of asystem for characterizing an oscillator. This system includes an inputport that receives an oscillation signal from an oscillator under test;an input port signal splitter that splits the received oscillationsignal into a first oscillation signal and a second oscillation signal;a first photonic signal processing branch circuit that processes thefirst oscillation signal to produce a first branch output signal; asecond photonic signal processing branch circuit that processes thesecond oscillation signal to produce a second branch output signal; adual channel signal analyzer that receives the first and second branchoutput signals to measure noise in the received oscillation signal; anda computer controller that controls the first and second photonic signalprocessing branch circuits and the dual channel signal analyzer tocontrol measurements of the noise in the received oscillation signal.

In one implementation of the above system, the first photonic signalprocessing branch circuit includes a first signal splitter to splits thefirst oscillation signal into a first branch signal and a second branchsignal; a photonic branch that receives the first branch signal andcomprises a laser that produces a laser beam, an optical modulator thatmodulates the laser beam in response to the first branch signal toproduce a modulated laser beam that carries the first branch signal, anoptical delay unit that transmits the modulated laser beam to produce adelay in the modulated laser beam, and an optical detector that convertsthe modulated laser beam into a detector signal; an electrical branchthat receives the second branch signal and comprises a voltagecontrolled phase shifter that receives the second branch signal and tochanges a phase of the second branch signal to produce an output signal;and a signal mixer that mixes the detector signal and the output signalto produce the first branch output signal.

These and other aspects and associated features and theirimplementations are described in greater detail in the drawings, thedescription and the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an example for an automated opto-electronicscross-correlation homodyne phase noise setup to illustrate varioustechnical features.

FIGS. 2, 3 and 4 show examples of cross-correlation phase noisemeasurement devices based on a shared optical modulator and a sharedlaser.

FIGS. 5, 6A, 6B, 7 and 8 show examples of phase noise measurementdevices that use optical polarization of light and Faraday rotatormirrors to reduce the physical lengths of fiber delay lines.

DETAILED DESCRIPTION

This application describes techniques, devices and systems for measuringphase noise in RF, microwave, or millimeter signals and forcharacterizing oscillators in RF, microwave, or millimeter spectralrange based on photonic components.

FIG. 1 shows an example for a phase noise measurement device based on anautomated opto-electronics cross-correlation homodyne phase noise setupto illustrate various technical features. This exemplary setup isimplemented via optical fiber serving as a long delay line. The dualhomodyne setup is then cross correlated at the signal analyzer to reducethe noise of each of the homodyne branches by averaging out noise thatis not correlated with the oscillator under test.

Phase noise measurements of RF, microwave or millimeter oscillatorsgenerating high purity electro-magnetic signals require low phase noisemeasurement setup. The present technique can be used to reduce the noisefloor of a single homodyne measurement setup by cross correlating thesignals of two measurement setups. The uncorrelated noise from each ofthe two measurement setups is averaged out at the signal analyzer. Thephase noise floor of the cross-correlated dual systems can be improvedby 5 log(N) (in dB units), where N is the number of averages.

Each of the two measurement setups is an electro-optic homodyne setupwith two signal branches. A signal splitter splits a received signalfrom an oscillator 101 into the two branches. The oscillator 101 undertest is coupled to the input port of the system which includes an inputport splitter 102. The two signal branches include two branch signalsplitters 102A and 102B, respectively. Each of the splitters 102A and102B splits the received signal into two signals for two branches.

The first signal branch is a photonic signal branch which includes ahigh-speed optical modulator (MOD) 111 or 121 to modulate a CW laserbeam from a laser 110 or 120 in response to the microwave/RF signal toproduce a modulated optical signal that carries the microwave/RF signal.The modulated optical signal is directed along an optical fiber whichserves as a signal delay line 112 or 122, allowing for efficientdiscrimination of the noise. The increase of the length of the fiber 112or 122 leads to an increased delay of the signal and reduces theclose-in phase noise of the setup. The photodetector (PD) 113 or 123converts the modulated light back into a microwave or RF signal which isthen amplified by an amplifier 114 or 124. The second signal branchincludes a voltage controlled phase shifter (VCP) 115 or 125 and asignal amplifier 116 or 126. A signal mixer 117 or 127 is used tocombine the two branches together to mix the signals from the twobranches to produce a beat signal. The VCP 115 or 125 controls the phasedelay of the signal in the second branch to produce a desired relativephase between the signals of the two branches at the signal mixer 117 or127, e.g., a 90 degree phase shift between the two signals known as thequadrature setting for the signal mixer 117 or 127 where the phase noiseis represented by the DC voltage in the beat signal. A dual channelsignal analyzer 130 is provided to receive the beat signals from the twomeasurement setups and to produce Fast Fourier Transform (FFT) on thebeat signals as FFT data. The cross correlation of the signals of twomeasurement setups effectively suppresses uncorrelated noise from eachof the two measurement setups without using a reference oscillator or aphase locking loop and provides a low noise measurement system formeasuring phase noise in an oscillator.

The optical delay line 112 or 122 provides a desired long optical delaythat is sufficiently large to reduce the noise floor of the device undera desired level. Different from a coaxial RF delay line which tends tosuffer from significant signal loss as the length of the delayincreases, the optical delay line can provide a long delay withrelatively small loss of light. Therefore, long optical delays, e.g.,with fiber in the range of kilo meters, can be achieved with fiberloops. The laser 110 which produces continuous wave laser light and theoptical modulator 111 collectively transform the oscillator signal fromthe oscillator 101 under test in the RF, microwave or millimeter domaininto the optical domain and the optical delay line 112 or 122 is used tointroduce the desired phase delay in the optical domain. Thephotodetector 113 or 123 then converts the phase delayed optical signalback to the RF, microwave or millimeter domain.

Such a system can be automated by using a voltage controlled phaseshifters (VCPs) and a computer controller 140. The VCPs 115 and 125 areused for the calibration (voltage to phase) of the setup and for tuningthe phase of the signal in the second branch at the mixer to bring themixer to the quadrature setting so the mixer output would be sensitiveto phase noise in the oscillator signal. The computer or microprocessor140 is used to carry out the measurement automatically. The computermeasures the calibration factor and put the mixer in quadrature. Thecomputer 140 also controls the signal analyzer parameters, such asfrequencies, the number of averages, the resolution, the bandwidth etc.In addition, the computer 140 can be used to generate plots of the phasenoise at the monitor and allows for saving the data.

In various implementations, the functions of the signal analyzer 130 andfunctions of the computer controller 140 may be grouped or separated invarious configurations. In some implementations, a signal processing andcontrol circuitry module may be implemented to provide the functions ofthe signal analyzer 130 and computer controller 140 as illustrated anddescribed herein. This circuitry module may be implemented withouthaving the same partition of functions as the signal analyzer 130 andcomputer controller 140. For example, if an analog-to-digital converter(ADC) is used instead of the signal analyzer 130 to receive the outputsfrom the mixers 117 and 127, the computer controller 140 can be used tocollect the digitized data from the ADC and calculate the FFT based onthe received data.

Following is a tuning and calibration procedure for thecross-correlation homodyne phase noise set-up in FIG. 1. The computercan be operated to perform this procedure automatically. The procedureincludes calibration, quadrature setting and phase noise measurements.

1. Calibration

In the calibration process, the computer 140 is used to send controlsignals to the VCPs 115 and 125 to sweep the bias voltages over the VCPs115 and 125. At the same time, the computer 140 is in communication witheach of the mixers 117 and 127 to record the mixer output voltageresponses through an analog-to-digital (A/D) conversion card.

Next, the computer 140 use stored calibration formulas for the voltagecontrolled phase shifters as a function of the VCP's bias voltage,φ(VVCP), to calculate the calibration responses for various VCP phasesto the mixer voltage (Δφ/ΔVmixer at Vmixer=0), for each of the twomeasurement setups. This completes the calibration process.

2. Quadrature Setting

Based on the calibration data, the computer 140 tunes the bias voltageof each VCP to shift the phase so that each mixer is at the zero DCoutput. This sets the mixers at quadrature so that the output of eachmixer is sensitive to the phase noise with low sensitivity to amplitudenoise at saturation.

3. Phase Noise Measurements

To measure phase noise of an input oscillation signal in FIG. 1, thecomputer 140 controls the signal analyzer 130 to set various operatingparameters, including the range of measurement frequencies, theresolution bandwidth, the number of averages and other parameters. Auser can control these parameters through the user interface software.After the operating parameters of the signal analyzer 130 are set, theinput signal is directed into the input port splitter 102 and dataacquisition is performed by using the computer 140 to retrieve outputvoltage fluctuations produced by the two mixers 117 and 127 and the FFTdata from the signal analyzer 130.

During the data acquisition, the computer 140 monitors the outputvoltage fluctuations produced by the two mixers 117 and 127. If theoutput voltage from a mixer drifts over an allowable range due to theoscillator frequency drift and/or the delay thermal drift, the computer140 sets the signal analyzer 130 on a pause mode to suspend the dataacquisition. Next, the computer 140 controls the VCPs 115 and 125 tobring the system to the quadrature setting again, and resumes the FFTmeasurements.

The FFT data retrieved by the computer 140 is then converted to a phasenoise spectral density using the calibration value measured during thecalibration and the fiber delay length factor. The data can be plottedon the screen of the computer 140, and optionally could be stored into afile.

The noise floor of the system could be improved by increasing N, thenumber of FFT averages. The noise floor drops as 5 log(N) (in dB units).

The above procedure describes only one of software modes of operation ofthe device in FIG. 1. Other modes for operating the device in FIG. 1includes using only one of the two homodyne setups, or measuring themixer voltage spectral density directly (for active/passive device phasenoise measurements). The device in FIG. 1 can be designed to allow auser to control the delay length of the fiber delay line 112.

The phase noise measuring device in FIG. 1 has the advantage of directphase noise measurements without relying on a second oscillator andphase locking. The RF carrier frequency range in such a device can bewide and may be limited by the RF amplifiers and VCPs. Someimplementations of the device in FIG. 1 can be used for RF carrierfrequencies between 6 to 12 GHz with a noise floor better than −110dBc/Hz at a frequency offset of 100 Hz, −140 dBc/Hz at a frequencyoffset of 1 kHz and −170 dBc/Hz for frequency offsets greater than 10kHz.

In the phase noise measurement device in FIG. 1 based on crosscorrelation between the two measurement branches, two sets of lasers,optical modulators and fiber delay lines are used in the two measurementbranches. Such lasers and optical modulators increase the device cost.The fiber delay lines tend to be bulky for some applications where eachfiber delay line may have a length of several kilometers. FIGS. 2, 3 and4 shows examples of designs that share photonic components betweendifferent measurement branches.

FIG. 2 shows an example that uses light at two different wavelengths λ1and λ2 to go through the same optical modulator and optical fiber delayline for effectuating the two separate photonic arms in the twomeasurement branches in the device in FIG. 1. The design in FIG. 2represents a microwave, RF or millimeter photonics cross-correlationphase noise measurement system that utilizes multiple delays over asingle fiber. This configuration delivers multiple optical signals, eachover a different optical wavelength, in a single fiber. Thisconfiguration reduces the required delay length by the number ofrequired delays, which can significantly reduce the system size. Passiveoptical couplers and WDM filters are used to route the optical signalsat different wavelengths. The design in FIG. 2 provides a simplifiedconfiguration for high order statistical calculations of signal momentsand correlation coefficients of multiple signals over the delay time.

In FIG. 2, a power splitter 201 is used to split the input oscillatorsignal from the oscillator 101 under test into two oscillator signals261 and 262. The oscillator signal 261 is directed into an opticalmodulator 220 and the oscillator signal 262 is further split intooscillator signals 271 and 272.

A light source 210 is provided to produce continuous wave laser light attwo different wavelengths λ1 and λ2. The light source 210 can beimplemented in various configurations, e.g., a dual mode laser thatproduces the laser light 212 wavelengths λ1 and λ2 or a light sourcethat includes two lasers respectively producing two laser beams atwavelengths λ1 and λ2. The CW light at the wavelengths λ1 and λ2 isdirected into an optical modulator 220 that receives the oscillatorsignal 261 from the power splitter 201 and modulates the received lightto produce modulated light at wavelengths λ1 and λ2 carrying theoscillator signal 261. The modulated light at wavelengths λ1 and λ2carrying the oscillator signal 261 is coupled into a single fiber delayline 230. The output light from the single fiber delay line 230 isdirected into a wavelength-selective optical device 240 that splits thelight into a first optical beam 241 at the wavelength λ1 and a secondoptical beam 242 at the wavelength λ2, both carrying the oscillatorsignal 261 with the delay caused by transmitting through the fiber delayline 230. The optical device 240 can be implemented in variousconfigurations. For example, the optical device 240 can be an opticalcoupler with two wavelength-division multiplexing (WDM) filters wherethe light is split into two portions to the two WDM filters. The firstWDM filter selectively outputs light at the wavelength λ1 whilerejecting light at the wavelength λ2, and the second WDM filterselectively outputs light at the wavelength λ2 while rejecting light atthe wavelength λ1. In another example, a single add-drop optical filtercan be used as the optical device 240 to selectively separate light atthe two wavelengths into two output beams 241 and 242 which can becoupled to separate fibers. The two output beams 241 and 242 are thenrespectively directed to two photodetectors 113 and 122 which convertthe received light back to the RF, microwave or millimeter domain. Thedetector output signals from the photodetectors 113 and 123 in FIG. 2function as equivalents of the two detectors output signals from thephotodetectors 113 and 123 of the two photonic signal paths of the twomeasurement branches in FIG. 1.

The oscillator signal 271 is directed to the VCP 115 which adjusts thephase of the signal 271 in response to a control signal from thecomputer 140 in FIG. 1. Similarly, the oscillator signal 272 is directedto the VCP 125 which adjusts the phase of the signal 272 in response toanother control signal from the computer 140 in FIG. 1. The outputsignal from the VCP 115 is then mixed with the detector output signalfrom the photodetector 113 at the mixer 117 and the output signal fromthe VCP 125 is then mixed with the detector output signal from thephotodetector 123 at the mixer 127. The output signals from the mixers117 and 127 are then directed into the signal analyzer 130.

Therefore, the optical modulator 220, the single fiber delay line 230,the optical device 240, the photodetector 113 in FIG. 2 form anequivalent of the photonic arm formed by the optical modulator 111, theoptical fiber delay line 112, and the photodetector 113 for the firstmeasurement branch in FIG. 1. The power splitter 201 and the VCP 115 inFIG. 2 form an equivalent of the other arm formed by the splitter 102Aand the VCP 115 for the first measurement branch in FIG. 1. Similarly,the optical modulator 220, the single fiber delay line 230, the opticaldevice 240, the photodetector 123 in FIG. 2 form an equivalent of thephotonic arm formed by the optical modulator 121, the optical fiberdelay line 122, and the photodetector 123 for the second measurementbranch in FIG. 1. The power splitter 201 and the VCP 125 in FIG. 2 forman equivalent of the other arm formed by the splitter 102B and the VCP125 for the second measurement branch in FIG. 1.

The design in FIG. 2 can be extended to a device with three or moremeasurement branches. FIG. 3 shows another example with threemeasurement branches that share a common optical modulator 320, and acommon fiber delay line 330. A light source 310 is provided to producecontinuous wave laser light 312 at three different wavelengths λ1, λ2and λ3. An optical device 340 is used to split the received light intothree beams 341, 342 and 343 at wavelengths λ1, λ2 and λ3, respectively.A third photodetector 350, a third VCP 380 and a third mixer 360 areprovided to form part of the third measurement branch that is operatedbased on the light at wavelength λ3.

FIG. 4 shows another example where an optical switch or splitter 410 isprovided allow different measurement branches to share the same laser110 and same optical modulator 111 without using separate lasers andseparate optical modulators as in the device in FIG. 1. The modulatedlight output by the shared optical modulator 111 is directed twodifferent fiber delay lines for two different measurement branches.

The above phase noise measurements measure phase noise through the useof delay lines. Microwave photonics optical links can provide longdelays by carrying the oscillator signal on an optical carrier overcompact, low loss, and long fibers. Reduction of the system noise flooris achievable with the help of cross correlation analysis viauncorrelated delays. Also, removing system artificial spurs associatedwith the use of long fiber delays may require multiple optical fiberdelay lines such as shown in the device in FIG. 1. The designs in FIGS.2 and 3 provide shared configurations where photonic parts of the deviceare shared by different measurement branches for the cross correlation,thus reducing the required total fiber length segments with a singlefiber. The single optical fiber shared by different measurement branchesis used to carry multiple microwave signals over different opticalcarrier wavelengths. Each of the signals is then extracted to be fed inseparate photodetectors for analysis. For example, dual delay line crosscorrelation setup could use a single fiber with a single coupler and twoWDM filters at the fiber end. Each signal is then down converted withthe help of a photodetector and a mixer. The two down converted signalsare then analyzed for cross-correlation (noise floor reduction) via adual channel signal analyzer.

Another example is the need for multiple fiber lengths for theelimination of artificial spurs due to the fiber delay. This could beachieved by having a few optical wavelengths carrying the sameRF/microwave signals over a single optical fiber. Each of thewavelengths is then coupled to a photodetector (after passing throughthe appropriate the fiber length/delay) via an optical coupler and a WDMfilter.

The above described techniques that use three or more measurementbranches shown in FIG. 3 can be used for complex analysis of signals,such as measurements of statistical moments and higher order correlationcoefficients. For example, a multi-channel signal analyzer that receivessignals from three or more measurement branches can be used to measurehigh order average values and correlations and, therefore, providebetter insight of the statistical distribution function of thestatistical parameter under test.

As discussed above, the long fiber delay line allows for reduced floornoise in phase noise measurements based on the delay discriminatordesigns presented in this document. In various implementations, eachlong fiber delay line may be of several kilometers and thus can be bulkyand occupy significant space in the device. Various applications prefercompact devices and it is desirable to reduce the actual lengths of thefiber delay lines while still maintaining the desired long delaysachieved through the fiber delay lines. One example is to use opticalpolarization property of light to direct light to pass through a fiberdelay line twice to cut the needed fiber length by one half. Severalexamples for such a design are provided below.

FIG. 5 shows an example of a dual channel cross correlation phase noisemeasurement device having two measurement branches based on an opticalmodule 510 that uses two orthogonal optical polarizations and Faradayreflectors to direct light through a fiber delay line twice. The twomeasurement branches in FIG. 5 are similarly structured as the design inFIG. 2 but the photonic delay line designs in FIGS. 5 and 2 aredifferent. In FIG. 5, the polarization-based optical module 510 isprovided to apply the oscillator signal from the power splitter 201 thatis originated by the oscillator 101 to modulate laser light in twoorthogonal polarizations to produce two modulated optical beams 511 and512 that carry the oscillator signal and undergo optical phase delays.The modulated optical beams 511 and 512 are directed into the twophotodetectors 113 and 123, respectively. The two modulated opticalbeams 511 and 512 can be of the same wavelength or two differentwavelengths as marked in FIG. 5 as long as the two beams are orthogonalin their polarizations for separating the two beams using apolarization-based photonic beam combiner (PBC) that combines andseparates light of the two mutually orthogonal polarizations. Threepower splitters 201 are used to split the oscillator signal from theoscillator 101 into four copies of the oscillator signal for the foursignal arms in the two measurement branches.

FIG. 6A shows one exemplary implementation of the polarization-basedoptical module 510 in FIG. 5. Two lasers 610 and 620 are provided toproduce two laser beams that are respectively directed into two opticalmodulators 612 and 622 in mutually orthogonal polarizations.Polarization maintaining (PM) fibers can be used to guide the two laserbeams from the lasers 610 and 620 in two orthogonal linear polarizationsto the two optical modulators 612 and 622 and PM fibers are also used atother locations where their mutually orthogonal polarizations aremaintained. The two optical modulators 612 and 622 are coupled toreceive two copies of the oscillator signal and modulate the oscillatorsignal onto the two optical beams from the lasers 610 and 620,respectively. The modulated optical beams output by the two opticalmodulators 612 and 622 are maintained at mutually orthogonalpolarizations and are directed to two optical circulators 614 and 624,respectively. The optical circulators 614 and 624 are optically coupledto photodetectors 123 and 113 and a polarization-based photonic beamcombiner (PBC) 630 as shown. More specifically, the optical circulator614 is coupled to receive the modulated optical beam from the firstoptical modulator 612 and to direct the light to the PBC 630 along theoptical path 631 (e.g., PM fiber) and to direct light from the PBC 630along the path 631 to the second photodetector 123; and the opticalcirculator 624 is coupled to receive the modulated optical beam from thesecond optical modulator 622 and to direct the light to the PBC 630along the optical path 632 (e.g., PM fiber) and to direct light from thePBC 630 along the path 632 to the first photodetector 113.

The PBC 630 is designed to combine the two orthogonally polarized beamsreceived from the optical paths 631 and 632 into a combined beam andcouples the combined beam into a fiber delay line 640, e.g., a singlemode fiber (SFM) delay line. The fiber delay line 640 is terminated at aFaraday rotator mirror (FRM) 650 which reflects light back to the fiberdelay line 640 to return to the PBC 630. The FRM 650 includes a Faradayrotator that rotates light polarization by 45 degrees in a single pathand a reflector that reflects light that transmits through the Faradayrotator back to transmit the Faraday rotator for the second time.Therefore, the polarization of the returned light from the FRM 650 isrotated 90 degrees. The two light beams that enter the PBC 630 via thetwo optical paths 631 and 632 remain orthogonal in polarization whenthey are reflected back to the PBC 630 by the FRM 650 but theirpolarizations are switched. As a result, the PBC 630 is designed toseparate the returned beams by their polarizations so that the returnedlight in the same polarization as the polarization in the optical path631 is directed by the PBC 630 to the optical path 631 and the returnedlight in the same polarization as the polarization in the optical path632 is directed by the PBC 630 to the optical path 632. Therefore, thelight that enters the PBC 630 via the optical path 631 is, afterreflection by the FRM 650, directed by the PBC 630 along the opticalpath 632 to the optical circulator 624 which, in turn, directs the lightto the first photodetector 113; and the light that enters the PBC 630via the optical path 632 is, after reflection by the FRM 650, directedby the PBC 630 along the optical path 631 to the optical circulator 614which, in turn, directs the light to the second photodetector 123.

FIG. 6B shows one example for implementing the PBC 630 by using apolarization beam splitting cube which reflects light in the firstpolarization in the optical path 631 and transmits light in the secondpolarization in the optical path 632.

Therefore, the above use of the optical circulators 614 and 624, the PBC630 and the FRM 650 allows the two signals from the two lasers 610 and620 to be combined into a single low cost single mode fiber 640 havingorthogonal polarizations. After traveling for one half of the requiredfiber length in a single channel, the light in the fiber 640 isreflected back using the FRM 650 which also rotates each of thepolarizations by 90 degrees. This compensates for any polarizationrotation along the fiber 640 and brings the signals back to the PBC 630with the original polarization states rotated by 90 degrees. The PBC 630then splits the two orthogonally polarized signals into the two separatefibers 631 and 632. The circulators 614 and 624 can be PM circulatorsthat allow coupling of these delayed signals into separatephotodetectors 113 (PD1) and 1234 (PD2). Each signal is then downconverted with the help of a respective PD 113 or 123 and a respectivemixer 117 or 127 (FIG. 5). The two down converted signals are thenanalyzed for cross-correlation (noise floor reduction) via the dualchannel signal analyzer 130.

Under the designs in FIGS. 5, 6A and 6B, the two separate signalspropagate twice within the fiber delay line 640 and the fiber lengthneeded for the desired delay is reduced to a quarter of the fiber delaylength that is otherwise needed for the two channels based on the designof a cross-correlation configuration of two separate fibers in FIG. 1.

FIG. 7 shows another example of the polarization-based optical module510 in which two FRMs 710 and 720 are used to provide two alternativeoptical paths for linking to the fiber delay line 640. An optical 1×2switch 730 is provided to connect the fiber delay line 640 to either oneof the two FRMs 710 and 720 to provide two different optical delays. Inthe example in FIG. 7, a second fiber delay line 740 is coupled betweenthe optical switch 730 and the FRM 710 so the optical delay for lightreflected by the FRM 710 is greater than the optical delay for lightreflected by the FRM 720. The optical switch 730 is operated to switchbetween fibers of two different total fiber lengths L1 and (L1+L2),respectively, and to effectuate fiber lengths are 2×L1 and 2×(L1+L2) dueto the double pass configuration by using the two FRMs 710 and 720.

The design in FIG. 7 can be extended to a multi-fiber length PNTS withmore than two FRM paths to eliminate artificial spurs at harmonics ofthe frequency related with the fiber length. A single 1×N optical switchis used to switch between N fiber paths with different delays andrespective FRMs.

The above use of the FRM can be implemented in a single channel PNTSshown in FIG. 8. In this example, the polarization of the laser light iscontrolled based on the polarization control in FIGS. 6A, 6B and 7without the optical circulators. The savings in this case is one half ofthe fiber length that would have otherwise required in a single channel.Similarly to the design in FIG. 7, two or more optical paths with FRMsand different delays can be implemented in FIG. 8 by using a single 1×Nswitch to switch between N fibers.

The single channel PNTS shown in FIG. 8 includes a signal analyzer 810that receives the beat signal from the mixer 117 and produces FFT dataof the received beat signal. A computer control 820 is used to sendcontrol signals to the VCP 115 to sweep the bias voltages over the VCP115. The computer control 820 is also in communication with the mixer117 to record the mixer output voltage responses through ananalog-to-digital (A/D) conversion card. Calibration and quadraturesetting procedures are performed before performing the phase noisemeasurements

The techniques and designs in FIGS. 5 through 8 can be used to providephotonic phase noise measurement systems that reduce the physical lengthof the fiber delay line based on the double pass configuration usingoptical polarization. For cross-correlation systems, this configurationdelivers together two signals, each having different (orthogonal)polarization, in a single fiber. A PBC is used for combining the twosignals into a single fiber with the orthogonal polarizations. PMoptical circulators, together with FRM, allow for separating the twopolarizations into two photodetectors after traveling twice in a quarterof the fiber length that would otherwise be required to achieve similarperformance of noise floor. This configuration can significantly reducethe physical size of phase noise measurement systems.

While this specification contains many specifics, these should not beconstrued as limitations on the scope of an invention or of what may beclaimed, but rather as descriptions of features specific to particularembodiments of the invention. Certain features that are described inthis specification in the context of separate embodiments can also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment canalso be implemented in multiple embodiments separately or in anysuitable subcombination. Moreover, although features may be describedabove as acting in certain combinations and even initially claimed assuch, one or more features from a claimed combination can in some casesbe excised from the combination, and the claimed combination may bedirected to a subcombination or a variation of a subcombination.

Only a few implementations are disclosed. Variations and enhancements ofthe described implementations and other implementations can be madebased on what is described.

What is claimed is:
 1. A device for measuring a phase noise in a signal,comprising: an input port that receives an oscillation signal from anoscillator under test; a photonic signal processing circuit thatprocesses the oscillation signal to produce an output signal; andcircuitry that receives and processes the output signal to measure noisein the received oscillation signal and controls the photonic signalprocessing circuit and measurements of the noise in the receivedoscillation signal, wherein the photonic signal processing circuitincludes: (a) a laser producing continuous wave laser light in a firstoptical polarization, (b) an optical modulator that modulates the laserlight to produce modulated laser light that carries the oscillationsignal and is in the first optical polarization, (c) a photonic beamcombiner that: i. receives at a first port the modulated laser light inthe first optical polarization along a first optical path from theoptical modulator, ii. directs the received modulated laser light in thefirst optical polarization to a second port, and iii. directs lightreceived at the second port in a second optical polarization orthogonalto the first optical polarization to a third port, (d) a fiber delayline coupled to the second port to receive light from the photonic beamcombiner, (e) a Faraday rotator mirror coupled to the fiber delay lineto reflect light back to the fiber delay line by rotating opticalpolarization by 90 degrees, (f) a photodetector coupled to receive lightfrom the third port of the photonic beam combiner to generate a detectorsignal, (g) a voltage controlled phase shifter that receives a copy ofthe oscillation signal and changes a phase of the copy of the oscillatorsignal to produce a phase-shifted oscillator signal, and (h) a signalmixer that mixes the detector signal and the phase-shifted oscillatorsignal to produce the output signal.
 2. The device as in claim 1,comprising: a polarization maintaining fiber that connects the opticalmodulator and the first port of the photonic beam combiner to maintainlight in the first optical polarization.
 3. The device as in claim 1,comprising: an optical switch coupled between the fiber delay line andthe Faraday rotator mirror to connect or disconnect a connection betweenthe fiber delay line and the Faraday rotator mirror; a second fiberdelay line separated from the first fiber delay line; and a secondFaraday rotator mirror coupled to the second fiber delay line to receivelight from the second fiber delay line and to reflect light back to thesecond fiber delay line by rotating optical polarization by 90 degrees,wherein the optical switch is further coupled between the fiber delayline and the second fiber delay line to connect or disconnect aconnection between the fiber delay line and the second fiber delay line.4. The device as in claim 1, wherein the circuitry includes: a signalanalyzer that receives and processes the output signal to measure noisein the received oscillation signal; and a controller that controls thefirst photonic signal processing circuit and the signal analyzer tocontrol measurements of the noise in the received oscillation signal.